1. Field of the Invention
The present invention relates to a rear wheel steering apparatus for a vehicle, and more particularly, it relates to a steering apparatus wherein left and right rear wheels of a vehicle are steered only by hydraulic pressure.
2. Related Background Art
In the past, various kinds of rear wheel steering apparatuses for vehicles have been proposed. Generally, the rear wheel steering systems are broadly divided into the following types: a system wherein rear wheels are steered by the steering force of front wheels transmitted to the rear wheels through a steering rod and the like, a system wherein the rear wheels are steered by the combination of the steering force of the front wheels transmitted to the rear wheels and the assist of hydraulic or mechanical power, and a system wherein the rear wheels are steered only by the hydraulic or mechanical power.
Among them, the system of the type last mentioned has an advantage over the other systems in that it has a greater degree of freedom in the control of the rear wheel steering so that the rear wheels can be steered even when the steering angle of the front wheels is zero. Further, among the systems of this kind, the system wherein the rear wheels are steered only by the hydraulic force or power generally comprises a hydraulic cylinder for steering the rear wheels by axially displacing a steering rod associated with the left and right rear wheels, and a control valve for controlling the supply and discharge of oil with respect to the hydraulic cylinder. An example of the system wherein the rear wheels are steered only by the mechanical force is disclosed in the Japanese Patent Laid-Open No. 57-44568.
Systems for steering rear wheels by hydraulic power alone have been characterized by various disadvantages and problems such as those addressed in the following discussion.
Firstly, in some systems, a centering spring is provided for returning the steering rod to a neutral position in order to return the steering angle to zero if a malfunction occurs. A centering spring cancelling mechanism is also provided for preventing a biasing force of the centering spring from acting on the steering rod during the normal operation. However, in this case, since all of the hydraulic cylinder, centering spring and centering spring cancelling mechanism are disposed on the steering rod, the steering rod must be longer, thus leading to a problem that the rear wheel steering apparatus is made large-sized.
Secondly, since there is provided a system for varying or changing a hydraulic pressure, i.e., main pressure fed from a hydraulic pump to the control valve to maintain a constant differential value between the main pressure and a negative pressure in order to cope with the variation in load for the rear steering operation during the running of the vehicle, the hydraulic circuit is made complicated.
Thirdly, since a feedback control is generally effected to monitor whether the rear wheels are steered in the controlled manner, a rear wheel steering angle detection sensor is provided for detecting the steering angle of the rear wheels on the basis of the axial displacement of the steering rod. However, in order to ensure the security, it is necessary to detect abnormality in the rear wheel steering hydraulic system early.
Fourthly, in the rear wheel steering apparatus of the relevant type and wherein the biasing force of the centering spring is always applied to the steering rod to return the steering rod to the neutral position if the malfunction of the hydraulic driving device occurs, there arose a problem that, when the rear wheel steering apparatus was driven, it was necessary to provide a force opposing to the biasing force of the centering spring, as well as a force required for steering the rear wheels. Further, there also arose a problem that, when the steering rod was displaced axially, the rod was struck against spring holders for the centering spring, thus generating noise.
Fifthly, in the aforementioned rear wheel steering apparatus, the rear wheels are steered by displacing the steering rod axially by controlling the pressure in the hydraulic cylinder of the rear wheel steering mechanism by means of a spool valve mechanism. Normally, the spool valve mechanism for shifting the steering rod is controlled by an attraction force of a solenoid or a pilot pressure, and a signal from a position sensor (disposed on the steering rod) for detecting the position of the steering rod is fed back to a controller for controlling the solenoid and the like, in order to shift the steering rod to a target position.
However, since there is no check whether the spool valve mechanism is being operated surely in response to the energization of the solenoid, for example, during the running of the vehicle, it is feared that the spool valve may be shifted wilfully even during the OFF condition of the solenoid, thereby displacing the steering rod, thus involving serious risk.
Sixthly, in the rear wheel steering apparatus of the type wherein the displacement of the steering rod is controlled by the spool valve incorporated in the hydraulic driving device, the spool valve is shifted to a predetermined position by the combination of the attraction force of the solenoid disposed in the hydraulic driving device and the biasing force of the centering spring tending to return the spool valve to the neutral position, thereby controlling the supply of the pressurized oil to the rear wheel steering apparatus to control the displacement of the steering rod.
However, because a space in which the solenoid is arranged is limited and the solenoid generates heat during the energization thereof, the rating of the solenoid cannot be high and the attraction force thereof is therefore limited. As a result, the driving force for the spool is small, thus leading to unstable operation of the spool.
Further, it is necessary to provide an adjusting means for adjusting for the irregularity in the attraction force of the solenoid and the irregularity in the biasing force of the centering spring, thus increasing the number of parts and making the hydraulic driving device complicated.
Seventhly, in the rear wheel steering apparatus of the type wherein the pressurized oil is supplied to the cylinder of the apparatus by controlling the operation of the spool valve by means of the solenoid and the like, in some cases, a centering mechanism such as a spring mechanism is provided for maintaining the steering rod in the neutral position when the driving force does not act on the rod.
In addition, various fail-safe mechanisms should be provided. When such a fail-safe mechanism is operated, the steering rod is returned to its neutral position by driving the centering mechanism.
The conventional fail-safe mechanism is generally of the type wherein the pressurized oil supplied from a hydraulic pump is relieved in a tank so as not to supply the oil to a control valve of the hydraulic cylinder. However, in the high pressure oil control utilizing an accumulator, since the fail-safe mechanism is not operated until the high pressure accumulated in a long oil line and in the accumulator is completely diminished, the desired fail-safe condition is not achieved as quickly as is desirable.
Eighthly, in the rear wheel steering apparatus of the type wherein the supply of the pressurized oil from the spool valve mechanism to the rear wheel steering apparatus is controlled by supplying the pressurized oil from a pressurized oil source to the spool valve mechanism through a supply line and by shifting the spool valve by means of the solenoid and the like, in order to shift the spool valve and the steering rod of the rear wheel steering apparatus quickly, an accumulator is often arranged in the supply line to supply high pressure oil to the cylinder chamber of the rear wheel steering apparatus.
However, if the oil pressure actually supplied to the cylinder chambers of the spool valve mechanism and the rear wheel steering apparatus is lower than the desired oil pressure, it will be hard to shift the spool valve and the steering rod quickly.
The control of the shifting of the spool valve and the steering rod with the high pressure oil has an advantage that the valve and the rod can be shifted quickly, but has a disadvantage that the control will be difficult when the oil pressure is decreased.